1. Field of the Invention
The present invention relates generally to an internal combustion engine induction system and more specifically to an induction system which uses multiple inlet valves per combustion chamber.
2. Description of the Prior Art
FIGS. 1 to 4 of the drawings show the construction and operational characteristics of the arrangement disclosed in in Japanese Patent Application First Provisional Publication No. 47-31724. In the induction system disclosed in this document each cylinder is provided with what shall be termed "a low speed" induction passage 1 and valve 2 which are arranged to introduce the incomming charge into the combustion chamber 4 in a manner that it swirls about the axis "A" of the cylinder and thus the combustion chamber. Viz., passage 1 is arranged to introduce the incomming charge into the combustion chamber with a flow vector which is essentially tangential with respect to the cylinder axis and which, as seen in FIG. 1, is arranged with respect to the axis as to maximize the flow component in a plane which is normal to the cylinder axis.
The system further includes what shall be referred to as a second "high speed" passage 6 for each cylinder. This passage is arranged to deliver the charge at a steeper angle into the the combustion chamber 4 and in manner which promotes charging efficiency rather than swirl therein. An inlet valve 7 controls communication between the combustion chamber 4 and the induction passage 6. The swirls produced by the two passages 1,6 tend to interfere with one another and thus suppress the formation of an excessively strong swirl under high engine speed operation.
A butterfly valve 8 is disposed in the high speed induction passage 6 and arranged to be closed at low engine speeds. The valve blade is supported on an axis 10.
The cam arrangement which controls the lifting of the exhaust and inlet valves is arranged to produced the lift characteristics shown in FIG. 3. Accordingly, with this arrangement as the induction volume under such low RPM modes of engine operation (e.g. idling) is low, the butterfly valve 8 is closed, the entire charge is forced to pass through the low speed passage 1 and thus enter the combustion chamber 4 at a velocity which generates a strong swirl therein which promotes stable combustion. As the butterfly valve 8 is closed the induction occurs as if the high speed inlet valve 7 were disabled and thus the induction characteristics are therefore controlled exclusively by the lift timing of inlet valve 2.
On the other hand, when the butterfly valve 8 is opened, as the lift of the "high speed" inlet valve 7 begins before that of the "low speed" one (2) the induction characteristics are controlled primarily by the timing of valve 7.
As shown in FIG. 2 this induction system includes a single exhaust passage 12 and valve 13. The lift timing of the high speed inlet valve 7 is selected in conjunction with the timing of the exhaust valve 13 to provide an overlap which promotes good engine respiration characteristics at high engine speed.
However, this arrangement has encountered the drawbacks that when the inlet and exhaust valves are lifted in the manner shown in FIG. 3, when the butterfly valve 6 is switched to its open position to permit induction to occur through both the high and low speed passages 1,6 as the lift timing of the high speed inlet valve 7 occurs before the low speed one the induction characteristics are as shown in FIG. 4 occur. Viz., the amount of charge inducted via the high speed passage 6 and the amount inducted through the low speed passage 1 are as shown in broken and solid line traces respectively. As will be noted most of the induction takes place through via the "high speed" passage. However, as the fuel is injected into the "slow speed" passage 1, due to the relatively small volume of air which is inducted therethrough the fuel tends to become insufficiently carburetted and leads to unstable combustion.
On the other hand, when the engine is operating under low speed conditions, the butterfly valve 8 is closed and the engine throttle valve 16 is only partially opened a realatively high vacuum in the order of 400 mmHg, develops in the induction manifold 17 downstream of the throttle valve 16 ( including the section downstream of the butterfly valve 18). It will be noted that this latter mentioned section of the induction system has a volume which corresponds to 15 to 20% of the combustion space (viz., the volume of the combustion chamber with the piston at TDC.
As the high speed inlet valve 7 opens before the slow one the pressure prevailing in the high speed induction passage 6 downstream of the butterfly valve 8 tends to vary as shown by the solid line trace in FIG. 6. Accordingly, just at the end of the exhaust phase of the engine when the high speed inlet valve is opened, a sudden flow of hot residual exhaust gas enters the high speed passage 6 at close to sonic velocity. This phenomenon apart from causing an undersirable high pitched whistling noise also causes the butterfly valve 8 to be subject to an undersirable impluse. Viz., as the butterfly valve in the cae of of a 1,800 cc engine (merely by way of example) has a surface area of from 6 to 8 cm.sup.2, the valve tends to be subject to a sudden force of the order of 3 to 4 Kg. This of course tends to have a detremental effect on the valve particularly the shaft 10 thereof and in some cases actually bends the same. Moreover, as the gases are still very hot, carbon deposits tend to form therein in a manner which tends to invite the butterfly valve 8 to "stick".
The above problem is even more pronouced in the event that the valves of the engine are lifted with a timing such as shown in FIG. 7. It is of course possible that a variable valve timing arrangement be used to shift the timing from that shown in FIG. 3 to that shown in FIG. 7. However, as will be appreciated this variability does not alleviate any of the problems set forth above.